A conventional cathode-ray tube normally comprises, between the cathode and the anode, three grids that are commonly known as G1, G2, and G3. Either the cathode or the grid G1 is effectively grounded, the CRT's video amplifier being connected to whichever of these two electrodes is not grounded, and the potential difference between the cathode and the grid G1 determines the beam current of the CRT. The grid G2 is maintained at a potential of about 200 to 800 volts, which is adjusted to establish a beam current cut-off point with a given G1 to cathode voltage. The focus grid G3 is normally maintained at a potential of about 6 kv, whereas the anode is maintained at a potential of about 25 kv. There is usually a capacitance associated with the anode that stores a substantial amount of energy. This capacitance may be the dag capacitance due to the CRT's external conductive coating, or it may be a filter capacitance in the power supply.
It is essential to proper operation of a CRT that the beam current should remain below a limiting value, since a very high beam current may result in burning of the CRT phosphor if the point of incidence of the electron beam on the phosphor remains stationary or moves at a low velocity.
Every CRT has the potential to arc via high voltage breakdown between the internal elements of the CRT. This breakdown is usually initiated by sharp burrs on the gun elements or loose particles within the CRT. The arc usually starts between G3 and the anode, where the highest potential difference exists. Normally, the arc then propagates down the CRT to G2. The G2 circuitry is usually of quite high impedance. Therefore, when the arc reaches G2 the voltage level of G2 goes very high, turning the beam on very hard. The arc can then propagate to G1 and the cathode. This may result in destruction of the video amplifier or the cathode itself.
The energy dissipated in the arc is usually quite high because of the charge stored on the capacitance associated with the anode. The currents generated by the arc have very short rise times, and the voltage rise times of elements connected to a high impedance are very high also (normally greater than 500 kv/.mu.s).
It is conventional to use a spark gap to help limit the voltage excursions at different CRT elements. A spark gap comprises two spaced apart electrodes. When a voltage exceeding a threshold voltage is applied to the electrodes of the spark gap, an arc will occur in the spark gap. In use of a spark gap to protect a CRT and associated circuitry, the spark gap is connected between a selected electrode, other than the anode, and a reference potential level, e.g. ground. When an arc forms, the voltage on the selected electrode of the CRT increases, increasing the voltage across the spark gap. When the voltage across the spark gap exceeds the threshold value, the spark gap fires and establishes a low resistance path between the selected electrode and ground, and the energy stored in the capacitance associated with the anode is shunted to ground. However, ionization of the gas in the spark gap path does not take place instantaneously, and there might be a significant delay between onset on a high voltage condition and firing of the spark gap. The length of the delay depends upon the magnitude of the potential difference, and the gap voltage can exceed the rated firing voltage of the spark gap for a short period of time without the spark gap's firing. This time may be sufficient for the arc to cascade down the CRT from one electrode to the next, and to cause large, fast voltage transitions to occur on these electrodes, possibly damaging the electrodes and circuitry connected to the electrodes.
A silicon controlled rectifier (SCR) is a semiconductor device having an anode, a cathode and a gate. In its normal mode of operation, the SCR is turned on (rendered conductive) by applying a small current to its gate, and the SCR remains conductive so long as the current flowing from its anode to its cathode remains above a holding value. An SCR will also be turned on if the potential difference between its anode and cathodes is increased at an excessive rate. For example, the Motorola 2N6241 has a dv/dt rating of 10 v/.mu.s (typical). It is generally accepted that the dv/dt rating of an SCR is a warning to the circuit designer to ensure that the SCR will not be exposed to potential differences changing at a rate greater than the quoted rating, since otherwise the SCR will be turned on otherwise than in response to a signal applied to its gate.